An interesting insect was discovered recently. A man in south central Minnesota in Watonwan county discovered some large caterpillars infesting some of the silver maples he was cutting down. They had created large tunnels about 3/8–1/2 inch wide in the wood. This insect, a type of carpenter moth, is known as a carpenterworm, Prionoxystus robiniae. In addition to maple, carpenterworms also attack a variety of hardwood trees including oak, elm, ash, black locust, cottonwood, and willow.
The larvae spend the winter in infested trees, pupating in spring. Because adult moths do not have chewing mouthparts, the larvae create exit holes from which brown pupae protrude. Adults are active in June and July. These stout-bodied moths have black and light gray mottled forewings. The female moths are considerably larger than males with a wingspan of 2 1/2 to 3 inches and are gray. The male's wingspan is about 2 inches and it somewhat resembles a sphinx moth.
Females lay eggs in cracks in the bark which hatch into small pinkish red larvae that tunnel into the sapwood. As they create galleries they keep openings available to regularly expel large quantities of sawdust which can collect in noticeable piles at the base of infested trees. They eventually can move into the heartwood as they get larger. These larvae can grow to be as large as two to three inches long and are yellowish-white caterpillars with brown heads. Carpenterworms are long-lived in Minnesota, taking three to four years to complete their life cycle.
Carpenterworms are found throughout the U.S. and southern Canada. Heavily infested trees are at greater risk of breaking in wind storms and become gnarled and misshapen. In some areas of the country carpenterworms are considered a serious tree pest. In Minnesota, they are not considered a serious landscape pest. While they certainly are present in urban and forested situations, they do not appear to be common enough to require management.
Originally published in Yard and Garden Line News, March 2008